Researchers butter up the old 'scratch test' to make it tough

MIT engineers used butter as a material for testing the fracture properties of materials. Credit: Photo / Pedro Reis

It might not seem like scraping the top of a cold stick of butter with a knife could be a scientific test, but engineers at MIT say the process is very similar to the "scratch test," which is perhaps the oldest known way to assess a material's hardness and strength  or, in scientific language, its resistance to deformation.

Using the scraping of butter as a starting point, the engineers launched a study to see if the age-old scratch test could be used to determine a material's toughness, or how well it resists fracturing after a small crack has already formed. The answer: The scratch test is indeed measuring crack resistance rather than strength and is valid on material samples of any size. This means that engineers now have a simple "new" test for assessing a material's fracture properties.

"Fracture mechanics has not reached the same level of pervasiveness in most engineering practice as strength theories, and this is due to the fact that it is difficult to determine fracture properties of materials, from soft clay to hard concrete," says Franz-Josef Ulm, the George Macomber Professor of Civil and Environmental Engineering (CEE) at MIT. "The test which we propose here is just this: a straightforward test for the engineering practice."

In a paper in Physical Review Letters that appeared online May 20, co-authors Ulm; Pedro Reis, the Esther and Harold E. Edgerton Assistant Professor of Civil and Environmental Engineering and Mechanical Engineering; and CEE graduate student Ange-Therese Akono  who is first author on the paper  describe their research and findings.

They performed laboratory scratch tests on paraffin wax, which is similar to butter but more stable at room temperature, Reis says, and used theory and mathematics to pare the process down to its essential components. They then created a mathematical model of the entire physical "scratch" process, which shows that the area of contact between the scratching implement and the test material is of primary importance in determining whether the scratch test is assessing strength or toughness.

They knew that when measuring a material's strength, the force required to make a scratch would always increase at the same rate as the contact area (width times depth) of the scratching tool.

But when measuring a material's toughness, the mechanics are complicated by the energy released when chemical bonds break as the new surfaces are created and a fracture grows. Because of this, the force does not increase at the same rate as the area of contact. Instead, the force exhibits a distinct scaling reminiscent of a fracture process  that is, a wider cut requires more force than a deeper one. (Specifically, the force increases at the same rate as the width times the square root of the depth.)

Back in the lab, the engineers changed the dimensions of the test to see if a wider scratching implement would require more force than a narrow one. It did. And that seemingly minor change in one dimension gave them their answer: The scratch test is assessing a material's fracture toughness, not its hardness nor strength properties. It assesses the hardness and strength only in cases where the area of contact between the scratching implement and the material is so small that a true indentation is made rather than a scratch. Now, knowing the width and depth of the scratch and the horizontal force, researchers can now determine the fracture toughness of a material.

"The advantage of a scratch test is that it works on both soft and hard materials and on very small samples," Akono says. "This method enables us to isolate brittle-crack propagation and neglect plastic deformation."

They confirmed their findings with additional tests on cement paste, limestone and steel.

"You might think that fracture, or how things break, is an old field of study," Reis says. "But it's relatively new compared to the tests of a material's hardness. Now, using the very old method of the scratch test, we have a relatively simple new means for measuring a material's toughness."

"The scalability of scratching for different probes and depths will open new venues for the miniaturization of the technique, which will help us understand fracture properties of materials at very small scales," Ulm says. "We also know  finally  that it takes less effort to make a narrow, deep cut in cold butter than a wide one. And that is science we can use at the dinner table."

New research at MIT has revealed for the first time the role of bone's atomistic structure in a toughening mechanism that incorporates two theories previously proposed by researchers eager to understand the secret behind ...

A new research technique developed by a team of researchers including Jeff Florando of DOE's Lawrence Livermore National Laboratory has shown that the mechanical properties of nickel and some of its alloys - such as their ...

Scientists and engineers are eager to understand the secret behind bone’s lightweight toughness so they can mimic it in the design of new materials, but previous studies have revealed a number of different strength mechanisms ...

Efforts to create a “hydrogen economy” to reduce U.S. oil imports will get a boost from a new laboratory at the National Institute of Standards and Technology that will evaluate tests, materials, mechanical properties ...

In work that could radically change how engineers search for new materials, MIT researchers have developed a way to test the mechanical properties of almost 600 different materials in a matter of days - a task that would ...

Recommended for you

A team of scientists has detected a hidden state of electronic order in a layered material containing lanthanum, barium, copper, and oxygen (LBCO). When cooled to a certain temperature and with certain concentrations of barium, ...

A team of researchers from the U.S., New Zealand and Norway has used computer simulations to predict several characteristics of the heaviest element, oganesson. In their paper published in the journal Physical Review Letters, ...

Researchers at the Center for Quantum Nanoscience within the Institute for Basic Science (IBS) have made a major breakthrough in controlling the quantum properties of single atoms. In an international collaboration with IBM ...

A team of researchers led by the Department of Energy's Oak Ridge National Laboratory has demonstrated a new method for splitting light beams into their frequency modes. The scientists can then choose the frequencies they ...

A team of researchers from several institutions in Japan has described a physical system that can be described as existing above "absolute hot" and also below absolute zero. In their paper published in the journal Physical ...

If they exist, axions, among the candidates for dark matter particles, could interact with the matter comprising the universe, but at a much weaker extent than previously theorized. New, rigorous constraints on the properties ...

0 comments

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.